The “wildly improved fan experience” part is true. Nats Park in DC, just to take the example I’m most familiar with, beats the hell out of any park from 40 years ago.
But the surge in attendance caused by the new wave of ballparks was modest compared to the surge that was sparked by free agency. Let’s look at per-game attendance over the respective periods:
From 1955 to 1975, MLB attendance was pretty stable - average per-game attendance (APGA just so I don’t have to write it repeatedly), by season, was almost entirely between 13,000 and 16,000 per game. In 1976, the last year before free agency became a thing for players not named McNally or Messersmith, APGA was 16,151. Then over the 1976-77 winter, free agency became real, all sorts of clubs had all sorts of new players, and 1977 APGA was 18,406, a 14% jump in one year. By 1979, APGA was 20,748, a 28.5% increase over 1976.
And it kept on going: in 1988, the last year before the SkyDome opened, APGA was 25,237; in 1991, the last year before Camden Yards opened, APGA was 27,002. In 1993, the year before Jacobs Field and The Ballpark in Arlington opened (and the year of the Last True Pennant Race, but I digress), APGA was 30,964.
At what point can you say the wave of new ballparks is driving attendance? Certainly one isn’t enough. I’d argue that you don’t have a critical mass of new ballparks until 1994, when you’ve got the four parks I just mentioned. But I’d be willing to go with two. So let’s take 1991 as the last year that free agency is the main driver, and give full credit to the new ballparks from then on.
Using that as our dividing line, free agency caused a 67% increase in per-game attendance over 15 years.
APGA in 2019 was 28,198, an increase of 4.4% over 1991.
OK, for whatever reason, per-game attendance has tailed off noticeably over the past two seasons. Let’s look at the 28 seasons from 1992-2019 collectively.
The best season was 2007, with APGA of 32,696, 21% better than 1991. The median season was 2004, with APGA of 30,075, an 11.4% increase over 1991.
In terms of attendance, free agency > new ballparks. Not even close.
ETA: cmkeller, I think this partially answers your question. I’ll try to get back to it later.